We have used a high-resolution motion analysis system to reinvestigate shape changes in guinea pig cochlear outer hair cells (OHCs) evoked by an external electric stimulation. In addition to longitudinal movements OHCs displayed bending movements when the external electric field gradient was applied perpendicularly to the cylindrical cell body. The amplitude of the bending movement was found to be as large as 0.7 micromoles. The specific sulfhydryl reagents, pCMPS and pHNPS, that suppress electrically evoked longitudinal OHC movements inhibit bending, thus the two mechanisms share the same underlying mechanism. OHC bending may result from an electrical charge separation that produces depolarization of the lateral plasma membrane on one side of the cell and hyperpolarization on the other. In the cochlea, OHC bending could produce radial distortions in the sensory epithelium and effectively influence the micromechanics of the organ of Corti. We have cloned and characterized cell lines from the organ of Corti (OC) and stria vascularis (SV). Cells from the OC and SV of the transgenic mouse, Immortomouse(TM), that harbors the temperature sensitive mutant of the A oncogene, when cultured at 33 degrees C cells proliferated quickly, doubling in number daily. When incubated at 39 degrees C they proliferated at a slower rate and progressively differentiated. Clonal cell lines were obtained from these cultures and we selected four clones from the OC and two from the SV which have been passaged more than 40 times. These cell lines can be distinguished by their characteristic cell shapes and differential labeling of cytoskeletal, epithelial, neuronal and other marker proteins. They constitute homogeneous cell populations that can be obtained at various stages of growth and differentiation. These cell lines will provide an unlimited source of homogeneous progenitors for the study of the mechanisms and signals involved in the control of differentiation and morphogenesis of the mammalian inner ear. They will also circumvent the need for primary cultures, which are extremely difficult to maintain and require large numbers of animals.